Pulp molding and pressure molding are processes for making soft or hard molded products from cellulosic fibers and/or mixtures thereof. The soft molded products which generally contain just cellulosic fibers include egg trays, apple fruit trays, flower pots and other containers. The addition of plastics or other cementitious fibers to the cellulosic fibers permit the manufacture of hard molded products. Generally, the hardness is obtained by finishing the molded products in a hot press.
For example, Japanese application, Feb. 5, 1973, No. 48-13848, discloses a method for making containers of wood pulp and polyolefin. The method includes making a mixture consisting of a polyolefin (10 to 300 parts by weight) and of wood pulp (100 parts by weight) and then forming a vessel from the mixture and then heating the vessel to a temperature above the melting point of the polyolefin.
However, applicant has found, as discussed in detail in Comparative Examples, that a single layer of a fused mixture of polyolefin and wood pulp does not result in a liquid resistant layer.
Generally, a pulp molding process involves the use of a male forming die covered with a wire mesh. The die passes through a stock chest holding a liquid suspension and the required thickness of solids is accreted on the forming surface. Liquid passing through the die is drawn off by a vacuum. Generally the liquid is water. The formed solid layer is removed from the forming die by various means.
The aforementioned male die can be a perforated rigid metal or plastic form, covered on the forming face with a fine wire screen and enclosed at the back to form a vacuum chamber. Thus, during the forming, water containing solids is drawn through the wire mesh where the solids are deposited, and flows into the vacuum chamber where it is drained off.
To separate most of the water from the shaped layer of solids, a press die is often used. A press die comprises a soft rubber or plastic bag which is fitted over the soft and wet shape. The bag is inflated to press, dewater and at the same time make the surface it contacts smoother.
To remove the dewatered, shaped layer from the forming die, a female transfer die can be used. The transfer die can be perforated rigid metal or plastic form which shape approximates the shape of the forming die. The transfer die is connected to both a vacuum and compressed air supply. During the transfer operation the perforated die is fitted over the pressed shape. While the die and the pressed wet shape are in close contact, vacuum is applied to the transfer die and compressed air is applied to the inside of the forming die. The pressed shape is thus taken off the forming die. The transfer die carries the pressed shape generally to an oven or some similar heating system where the pressed shape is blown off the die by a gentle blast of compressed air. After the article passes through the oven it emerges as a finished product.
A pressure molding process is described in the section titled Description.
However, there is a problem associated with making inexpensive, liquid holding containers of cellulosic fibers and polyolefin fibers. If not enough polyolefin fiber is used, some cellulosic fibers are not encapsulated and too much of the contained liquid wicks through to the outside surface making the container unsatisfactory. To avoid the foregoing the amount of polyolefin fiber used in the mixture is increased but then the cost of the container increases. The latter occurs because generally the cellulosic fibers are less expensive then the polyolefin fibers. Thus, the function of the cellulosic fiber is to minimize the cost of the container whereas the function of the polyolefin fiber is to make the container resistant to liquids. Applicant's invention overcomes the foregoing problem.
Also, oil containers, which are internally polyethylene coated, can be formed by spiral winding of polyethylene coated cellulosic stock. This stock is made by extrusion coating. Containers prepared by spiral winding can have leakage at the seams of the metal closures as well as the seams of the spiral wind. To compensate for such leakage the current practice is to add additional liquid to allow for wicking at the seams and thereby meet weight and measure test procedures. Applicant's invention overcomes this problem because of the reduction in the amount of seams while maintaining an impervious inner coating.